In most land-based applications, navigation is often aided by in-place infrastructure such as GPS, radio beacons or a priori maps. Navigation and mapping underwater is difficult because among other things, wide-coverage underwater GPS-equivalents do not exist and large portions of the sea bed are still unexplored.
Current techniques for underwater navigation use publicly available bathymetry maps. However, these maps are relatively coarse and unsuitable for precision navigation. Other sonar-based navigation systems rely on positioning schemes that use the sonar data itself. For example, on-the-fly acoustic feature-based systems attempt to use sonar to detect naturally occurring landmarks. Other solutions to the navigation problem include deploying low-cost transponders in unknown locations thereby enabling range-based measurements between the vehicle and transponder beacon. However, these transponders are often deployed at locations that are at great distances from each other, and often only partially observable because of the range-only information. Thus, these technologies are unsuitable for navigation across small vehicle paths.
Recent technologies permit navigation of underwater terrain relative to a prior map of the terrain. Such technologies use synthetic aperture sonar systems for generating images of the terrain, which are then compared against a prior image associated with the terrain. Underwater vehicles may then be able to navigate on the terrain relative to their location on the map. These technologies, however, suffer from a plurality of deficiencies including the amount of power consumed, size and shape of the systems. Additionally, the performance of such navigation systems dramatically decreases as transmitter frequencies increase and wavelengths decrease, or as range increases.
Most sonar equipment, such as sonar projectors and receivers, have frequency responses that include nulls. Engineers typically design the operating bands of such sonar projectors and receivers to be between the nulls (especially for sonars being operated off resonance). For example, a sonar receiver may be designed to operate in several operating bands, such as a low frequency (LF) band and a separate high frequency (HF) band, with an intentionally engineered null separating the two bands. However, as increasing pressure is applied to the sonar elements and the elements compress, the frequency response may change. The nulls may move into the operating band, thereby degrading the performance of the sonar equipment. One typical solution is to design the sonar to operate within a depth band (e.g. surface to 3000 m, 3000 m-6000 m, etc.). However, this is not feasible for applications that operate in a range of depths and ocean environments. In such varied environments, several versions of the sonar equipment would be required for each of the depth bands of the varied environments, driving up weight and cost. Thus, there exists a need for a variable-depth sonar system.
Inertial navigation is a common method of navigation underwater, but it suffers from errors that grow with time. Noisy acceleration measurements lead to velocity estimates with integrated errors and position estimates with doubly integrated errors. These errors can be alleviated by explicitly measuring velocity (even occasionally), thereby allowing the inertial system to remove biases in the velocity estimate and significantly reducing the rate of position error growth. State of the art Doppler sensors typically consist of multiple clustered pencil beam transducers. Transmitting signals in multiple directions allows the cluster of transducers to measure the velocity along multiple vectors (typically not orthogonal, although they could be). Those multiple velocity vectors can then be fused to provide a true three-dimensional velocity estimate.